A frequency-shift keying (FSK) modulation scheme that assigns values (“0” and “1”) of a digital signal to mutually different frequencies to modulate the digital signal is known as a digital communication modulation scheme. Patent Literature (hereinafter, referred to as “PTL”) 1 discloses an example of a demodulation scheme for a signal modulated using the FSK modulation scheme.
FIG. 1 is a block diagram illustrating a receiving apparatus (multilevel frequency-shift keying demodulator) disclosed in PTL 1. The receiving apparatus illustrated in FIG. 1 includes: antenna 1; primary demodulator 2, which conforms to a frequency hopping scheme; and secondary demodulator 3, which conforms to an M-FSK scheme (M-FSK demodulator). Primary demodulator 2 includes: mixer 5; frequency synthesizer 6; and hopping pattern generator 7. Secondary demodulator 3 includes: band-pass filter (BPF) 8; analog-to-digital (AD) converter 9; fast Fourier transformer (FFT) 10; maximum value selector 11; and decoding circuit 12.
Mixer 5 mixes a spread-spectrum signal received via antenna 1 and amplified by an amplifier (not illustrated) with a hopping local signal from frequency synthesizer 6 in synchronization with each other and despreads the spread-spectrum signal, thereby generating a primary demodulation signal. BPF 8 removes an unnecessary signal from the primary demodulation signal outputted from mixer 5. AD converter 9 converts the analog primary demodulation signal outputted from BPF 8 into a digital signal.
FFT 10 extracts the digital signal outputted from AD converter 9 in each pre-set time window and performs a fast Fourier transform and simultaneously detects a plurality of frequency components (FFT signal) of the digital signal.
Maximum value selector 11 detects a change in frequency components indicating a plurality of maximum amplitude values from the FFT signal outputted from FFT 10 and generates codeword data signal S1 having a plurality of codeword chips as a received signal in accordance with the change in frequency components. Moreover, maximum value selector 11 detects a maximum likelihood between codeword data signal S1 and codeword pattern data signal S2 of a plurality of kinds pre-set in accordance with bit pattern types of secondary demodulation data and selects codeword pattern data signal S2 having a highest matching degree with the codeword data.
Decoding circuit 12 decodes the codeword pattern data signal selected by maximum value selector 11 into a digital signal formed of a predetermined number of bits and outputs the digital signal as a secondary demodulation data signal (demodulated data).
As described above, the multilevel frequency-shift keying demodulator disclosed in PTL 1 performs a fast Fourier transform on an FSK modulation signal to detect a plurality of frequency components included in the modulation signal, thereby obtaining a data signal based on the plurality of frequency components. The technique disclosed in PTL 1 uses no envelope detector and requires no use of a plurality of band-pass filters. Thus, the technique disclosed in PTL 1 enables fast and accurate demodulation without variation in characteristics.